Liquid crystal cells comprise a layer of liquid crystal material between two glass plates, at least one of which is transparent, which have a conductive layer or segments thereon to act as electrodes. In operation, the liquid crystal molecules, which are initially aligned in a certain direction with respect to the plates, respond to the presence of an electric field either by re-aligning themselves in a different direction, or by dynamic scattering. This change in alignment affects the passage of light through the cell and thus there is a change in contrast between the "on" and "off" states which can be employed in a display. By suitable patterning of the display, digital or other information can be made visible.
Certain types of liquid crystal molecules, those having negative dielectric anisotropy, generally employed for some types of cells such as dynamic scattering cells, deformation of aligned phase cells (DAP), reflective storage mode devices and the like, are most often aligned in the direction perpendicular to the glass plates which comprise the cell walls in the off state. Other liquid crystal molecules, having positive dielectric anisotropy and employed for example in twisted nematic field effect liquid crystal cells and guest-host cells, are normally aligned in a direction parallel to the cell walls. It will be apparent that any deviation from perfect off-state, initial alignment, either due to different angles of alignment or because of different angles between different molecules, will become manifest as visual defects in the cell or as a loss of contrast between the on and off states.
Various methods have been proposed heretofore to maximize the original perpendicular alignment of negative anisotropy liquid crystals.
Homeotropic aligning agents have been added directly to a liquid crystal composition, but this method has the disadvantage that the aligning agents are difficult to purify and the impurities adversely affect the lifetime of liquid crystal devices. Further, when filling a liquid crystal cell through a single fill port, a chromatography effect tends to concentrate the dopant around the fill port and the edges of electrode patterns, thereby giving rise to inhomogeneities in composition throughout the cell and thus to non-uniform alignment.
The cell walls have also been coated either with surfactants or with silane coatings such as dimethyl octadecyl-3-aminopropyl trimethoxysilyl chloride, which induces perpendicular alignment of the liquid crystal molecules with respect to the cell walls.
It has also been proposed to coat the electrode plates with thin layers of polymers produced by exposing the plates to a glow discharge in the presence of organic or organometallic compounds, including hexamethyl disiloxane, methyl methacrylate and isobutylene. The latter methods induce effective alignment, but they cannot withstand elevated temperatures, needed particularly for glass frit sealing of liquid crystal cells. Glass frit sealing requires temperatures of over 500.degree. C, usually about 525.degree. C, in order to form a hermetic seal between the electrode plates.
Thus a method of aligning liquid crystal molecules perpendicular to the electrode plates which will withstand a frit sealing step would be highly desirable.